1. Field of the Invention
The present invention relates generally to a frequency mixing apparatus, and in particular, to a frequency mixing apparatus in which a P-channel metal oxide semiconductor (PMOS) transistor is coupled to an N-channel metal oxide semiconductor (NMOS) transistor in a cascode configuration and a local oscillator (LO) signal is applied to the bulks of the PMOS and NMOS transistors so that an input signal applied to their gates is mixed with the LO signal.
2. Description of the Related Art
Typically, a mixer is used for frequency conversion when designing a radio frequency (RF) transceiver in a mobile communication system. Since this mixer relies on the non-linearity of diodes or metal-semiconductor field effect transistors (MESFETs), a large number of harmonic waves and intermodulation distortion (IMD) signals are produced when an RF or intermediate frequency (IF) signal and a LO signal are applied.
FIG. 1 is a circuit diagram of a conventional frequency mixer. Referring to FIG. 1, the frequency mixer is a kind of multiplier. An output signal Vout is the product of an input signal Vin and a LO signal VLO. That is, the output signal Vout is a signal having the voltage level of the input signal Vin at the frequency of the LO signal VLO.
The frequency mixer includes three differential amplifiers. A differential amplifier with two transistors Q1 and Q2 generates an output proportional to the difference between inputs Vin, that is, the difference between the collector currents of the transistors Q1 and Q2, IC1–IC2. Specifically, as the input signal Vin swings between positive (+) and negative (−) values, the collector currents IC1 and IC2 flowing through the transistors Q1 and Q2 also swing.
The collector current IC1 of the transistor Q1 is the output current of another differential amplifier with transistors Q3 and Q4. The collector current CC2 of the transistor Q2 is the output current of a third differential amplifier with transistors Q5 and Q6. These two differential amplifiers generate outputs proportional to the difference between LO signals VLO. More specifically, along with the swing of the LO signal VLO, the outputs (IC3–IC5) and (IC4–IC6) of the differential amplifiers also swing in opposite directions. Consequently, each of the transistor pairs Q3 & Q4 and Q5 & Q6 can be replaced with complementary switches. When viewed as switches, the two transistors Q3 and Q6 are turned on/off simultaneously and the two transistors Q4 and Q5 are also turned on/off simultaneously.
Current IL1 flowing through a resistor RL1 is the sum of the collector current IC3 of the transistor Q3 and the collector current IC5 of the transistor Q5. Current IL2 flowing through a resistor RL2 is the sum of the collector current IC4 of the transistor Q4 and the collector current IC6 of the transistor Q6.
Therefore, the differential amplifier with the transistors Q1 and Q2 operates depending on the operations of the differential amplifier with the transistors Q3 and Q4 and the differential amplifier with the transistors Q5 and Q6. This means that the output signal Vout is equivalent to the waves of the input signal Vin at the voltage level of the LO signal VLO. This type of frequency mixer is typical and its output signal Vout is expressed as Vout=RL/RE·Vin·VLO.
The above conventional frequency mixer mixes frequencies by controlling the switching of the input signal by means of the LO signal. It exhibits the drawbacks of very poor linearity, unavailability in a low supply voltage range, great power consumption in the case of increasing the supply voltage range, and the increase of high direct current (DC) offset voltage level caused by leakage current of transistors.